This invention relates generally to dynamic focus circuits for cathode ray tube (CRT) displays and specifically to parabolic voltage waveform generators for producing appropriately shaped voltage waveforms for dynamic focussing.
In CRTs, the radius of deflection of the electron beam is not the same as the radius of curvature of the faceplate. As CRTs became "shallower", their overall depth decreased resulting in the deflection center being moved closer to the faceplate. Also, the radius of curvature of the faceplate was increased. Under these conditions, the electron beam traverses a much longer path to the extremities of the faceplate than it does to the center of the faceplate. In rectangular shaped tubes, the problem is exacerbated at the outermost points on the CRT "diagonals" where the electron beam travel path is longest. It is well known that the bundle of electrons in the beams tend to spread with distance travelled, leading to defocussing. There are many prior art circuits for providing dynamic focussing voltages to the electron beam to compensate, or attempt to compensate, for this defocussing effect. Since defocussing is a function of the square of the distance from the center of deflection, the dynamic correction frequency applied to the CRT focus electrode is generally parabolic in shape. Complete correction requires both a horizontal and a vertical component. In conventional television displays, the vertical parabolic correction frequency is 60 Hz and the horizontal parabolic correction frequency is 15,750 Hz.
The usual approach to obtaining the required parabolic voltages is to use the well known double integration process. In such a process, a pulse voltage of proper frequency is applied to a suitable capacitor for developing a sawtooth voltage waveform which, in turn, is applied to another suitable capacitor to form a parabolic voltage waveform. Because there are DC currents involved, a tilt in the wave shape of the parabola occurs unless the circuits are capacitively coupled. While the circuits appear simple, up to 70 percent of the energy in the pulse is lost in each integration. Thus, a relatively high voltage pulse is required to produce a usable parabolic voltage wave shape. Such circuits tend to become complicated and invariably require active devices to achieve adequate adjustments for parabola gain and tilt, which adjustments are required in a precise dynamic focussing system. A major drawback is that because of the AC coupling and the high pulse voltage requirements, the prior art dynamic focus correction circuits do not lend themselves to integrated circuit fabrication.
The rapidly increasing use of computers has given rise to a great need for precision in the associated CRT displays. In most applications it is imperative that the CRT be capable of very high resolution and of maintaining sharp beam focus throughout the entire display area. This major difference in the nature of the computer data, as distinct from the video content of most commercial television programming, imposes much more demanding requirements on "edge" focus of the CRT beam. It is precisely in this area however, that the CRT beam experiences the greatest defocussing action because of the longer travel path. There is thus a need in the art for a relatively simple parabolic voltage generating circuit that may be readily fabricated in integrated circuit form, readily controlled as to amplitude and tilt, and precisely controlled as to frequency.